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State Attorney General Eric Schneiderman vowed Thursday to thwart an attempt by deep-pocketed backers of mayoral candidate Joe Lhota to overturn New York’s $150,000 cap on campaign contributions and spend millions boosting the Republican underdog’s campaign.

“Attorney General Schneiderman believes that New York State’s campaign finance rules are critical to ensuring that every New Yorker’s voice is heard,” said AG spokesman Matt Mittenthal. “The attorney general will vigorously defend the constitutionality of New York’s law.”

A lawsuit filed by the New York Progress and Protection PAC claims the group had “multiple” donors prepared to give more than $150,000 each to help promote Lhota’s candidacy.

Lhota defended the effort, which would represent an independent expenditure on his behalf.

“I don’t understand why anyone would want to limit an American’s First Amendment rights . . . Freedom of speech is in my view the most important part of our Constitution,” he said.

But Democratic front-runner Bill de Blasio charged a right-wing plot was afoot.

“This is clearly an effort to advantage my Republican opponent and to do so by bringing in more and more money flooding into the political process from very wealthy right-wing donors . . . and trying to drown out the voices of the people,” he said.